We have ample evidence that hadrons are made of constituent quarks: mesons of a quark and an antiquark, baryons of three quarks, but QCD leads us to expect a richer spectrum.
Scalar mesons are the Higgs sector of QCD. They are responsible for the masses of all light hadrons, made of up, down and strange quarks. What these states are will be described.
It is in this scalar sector that four-quark, molecular and glueball states most naturally arise in QCD, in addition to plain quark-antiquark mesons. Can experiment tell which is which?
A feature of the lightest of them is their close relation to their decay channels. This allows notions of S-matrix theory to distinguish their make-up. How high statistics data suitably analyzed make this mathematical rule applicable in practice will be explained. A rule that could be applied to other mesons and excited baryons, where fine enough data exist.